diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 5ccdedc76ca6..12f3d8b9e754 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -199,17 +199,12 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             }
         }
         if !attr.is_doc_comment()
-            && attr.get_normal_item().path.segments.len() == 2
-            && attr.get_normal_item().path.segments[0].ident.name == sym::diagnostic
+            && let [seg, _] = attr.get_normal_item().path.segments.as_slice()
+            && seg.ident.name == sym::diagnostic
             && !self.features.diagnostic_namespace
         {
             let msg = "`#[diagnostic]` attribute name space is experimental";
-            gate_feature_post!(
-                self,
-                diagnostic_namespace,
-                attr.get_normal_item().path.segments[0].ident.span,
-                msg
-            );
+            gate_feature_post!(self, diagnostic_namespace, seg.ident.span, msg);
         }
 
         // Emit errors for non-staged-api crates.